Ixii GENERAL SUMMARY OF SCIENTIFIC AND 



atmosphere were removed, the brightness of the sun's disk 

 would be uniform, and 3.83 times that of the centre of the 

 disk at present. Moreover, the total amount of light would 

 be increased 4.G4 times. 



Curtis has published a method of showing the phenomena 

 of extraordinary reflection. Upon a horizontal circular stage, 

 movable around the axis and adjustable in height, a crystal 

 of Iceland spar well polished is placed. A beam of light 

 falls on the crystal at such an angle that after refraction 

 and reflection within it the beam shall pass from it, making 

 the same angle on the other side of the normal. Five im- 

 ages of the opening through which the beam of light comes 

 will be seen, one formed by reflection at the upper surface, 

 and the other four by double reflection within the crystal. 



Williams has made a photometric investigation into the 

 intensity of twilight when the sun is at various distances be- 

 low the horizon. The percentage of error in the instrument 

 employed was about three. The results of the photometer 

 readings were reduced to the light given by a standard 

 candle as unity, when burning at a distance of one meter 

 from the disk. By a graphical construction of the actual 

 results a curve was obtained, and a table deduced which 

 gives the percentage of light, compared with that at sunset 

 as unity, for any number of minutes after sunset up to 34. 

 At 1 minute it is 0.95; at 10 minutes, 0.290 ; at 20 minutes, 

 0.064 ; at 30 minutes, 0.009 ; and at 34 minutes it is 0.004. 



Crosby, also, in the Massachusetts Institute of Technology, 

 has made some photometric determinations of the light of 

 the sky at different distances from the sun, adjusting the 

 mirror and lens which were employed so that the sun's im- 

 age would fall on the disk, and then measuring the intensity 

 of the light at regular intervals thereafter. In some cases 

 this method was reversed. The results represented graphic- 

 ally show a logarithmic curve, when the intensities are taken 

 as ordinates and the natural sines of the sun's angular dis- 

 tance as abscissa?. The author calls attention to the mete- 

 orological importance of his results. 



Giraud-Teulon has discovered a new method of measuring 

 distances optically, and has constructed a telemeter based 

 upon it. A double image of the object is produced by a 

 division of the eye-piece, one half moving by the other by 



